The present invention relates to an apparatus and method for providing separate dedicated and shared control channels in a communications system and, more particularly, in a forward link of a Code Division Multiple Access (CDMA) communication system.
Typically in communication systems such as CDMA and, more particularly, CDMA evolutions such as WCDMA, dedicated channels are required for sending data and also for control of the system. Within the forward link of such systems, a single channel is used to carry the information to be transmitted and characteristics of the transmitted information are conveyed on another separate forward control channel. Each of these channels occupies a Walsh code from a finite set of available Walsh codes. To keep the system from becoming Walsh code limited it is important to conserve Walsh code resources. The allocation of these dedicated channels for each user typically requires rapid shuffling between dormant and active states to free up Walsh code resources. Problems that arise from this rapid shuffling include too few bits available, too few users, too much power, too much reverse link automatic repeat request (ARQ), latancy, or too much reliance on using a data channel such as a shared channel (SHCH) for layer 3 information (e.g., switching of mobile stations between active and hold states) and ARQ. The shared channel SHCH is the forward channel used to send data packets to users in the active state and can reach very high peak data rates by using over 80% of the available Walsh code resources. However, the Walsh code resources are limited in the conventional systems because of the relatively small spreading factor sizes required to support the required payload and coding bits sent on the SHCH. Thus, a larger size spreading factor is critical for affording less power and more users to be supported, which are possible if bits can be more efficiently allocated.
Stated differently, the desire is to use as little of the Walsh Code resource as possible for a dedicated channel that, therefore, requires using a large spreading factor (a Walsh code of relatively large size such as 256 for CDMA2000 or WCDMA). The smaller the Walsh code resource used, the larger the number of dedicated channels that may be supported, which helps make rapid shuffling requirements less stringent. With a large spreading factor it is difficult to support many bits for a given frame duration, which is typically small (e.g. 3.33 ms) to match that of a data channel like the SHCH, and to optimize over the air transmission and scheduling. Using smaller spreading factors not only uses more Walsh code resources, but requires more transmit power for a given reliability level and reduces system capacity. Also it is possible to achieve more bits with a large spreading factor by using higher order modulation such as 8PSK but, again, at the expense of more transmit power.
There is therefore a need for a method and apparatus that affords more efficient allocation of bits and Walsh codes resources in a spread spectrum communication system.